Microelectromechanical systems (MEMS) have been developed for miniaturizing many different systems, such as scientific instruments or real-time monitoring devices. Many MEMS devices resemble integrated electronic circuits in that they are actually components that must be combined with other components to achieve a desired function. Unlike integrated electronic circuits, MEMS frequently must connect not just electrically to other components, but also by other physico-chemical parameters, such as optically and fluidically.
Electrical connections for miniaturized systems, such as integrated circuits, have benefited from extensive technical development, both to improve the connection (e.g. decrease form factor, decrease resistance, increase tolerance to extreme conditions) and to improve manufacturability, primarily to make formation of the connection amenable to automation.
MEMS are a less mature technology, and optical and fluidic connections from MEMS to other components remain very problematic. Improvements are needed both to improve the connections and to make them more manufacturable. In the case of optical connections, extensive effort has been expended to automate the connection of just fiber optics. The development of all-optical communications networks, however, requires the integration of many, diverse optical components, such as spectral filters, lasers, diffraction gratings, beamsplitters, and photodetectors. The connection and assembly of many of these components is still a manual process. Fluidic connections have proven the most problematic. Fluidic connections can have many of the same requirements that other MEMS connections do, such as micrometer precision of placement, and rigid and strong mechanical attachment. However, fluidic connections must conform to the edges of the fluidic passageways, making a water-tight seal without occluding the passageways. Furthermore, the seal must be able to withstand pressure of tens of pounds per square inch (p.s.i.) for low-pressure systems and tens of thousands of p.s.i. for some higher pressure systems. Furthermore, fluid connections must be compatible with the fluids to be transported. The materials of the fluid connections must be inert—they should not dissolve in, imbibe, or react with the fluid or chemicals dissolved in the fluid; nor should dissolved chemicals adsorb to the surfaces of the connection.
Fluid connections sometimes also must achieve stringent requirements for dead volume (the volume of the connection), void volume (volumes that extend out of the needed connection), and dispersion (defined later). Fluidic connections with large dead volumes can greatly increase the total volume of a system, contrary to the goal of miniaturization. Dispersion is the tendency of a fluidic system to degrade chemical concentration gradients. For example, if a chemical dissolved in a flow is suddenly increased, then the increase in concentration can be considered as a step that flows down the fluidic channel. Dispersion acts to reduce the steepness of the step—a sudden increase in concentration is turned into a more gradual gradient due to dispersion. One common contributor to dispersion is “unswept” or “void” volume. This is a volume of fluid in the interconnect that is outside the main flow through the interconnect. For example, a crevice between the ends of the walls of two channels that are joined end-to-end will contain a volume of fluid that is stagnant, even if fluid flows through the channel. Similarly, any sudden expansion or contraction of the fluid channel diameter will produce corners where the fluid flows more slowly, increasing dispersion.
Void volumes also result in “carry-over” when different fluids are passed sequentially through a fluidic system. Carry-over results in contamination of fluids by fluids that previously passed through the system. Such contamination is extremely problematic for analytical systems that must have, for example, large dynamic range or sensitive detection.
Connection of MEMS microfluidic channels to external fluid reservoirs frequently includes the attachment of microcapillaries to the MEMS microfluidic channels. This is done by a variety of techniques, such as gluing or use of fittings traditionally used in liquid chromatography. Examples of microfluidic connections include the “sipper chip” technology described in U.S. Pat. No. 5,779,868, the NANOPORT™ components available from Upchurch Scientific, Inc. (Oak Harbor, Wash.), and various connections in the CAPTITE™ and CHIP-TITE™ series developed at Sandia National Labs. A more experimental system that attempts a more comprehensive solution to multi-type connections for MEMS is described in Galambos et al. 2001, Proc. Of 2001 Amer. Soc. Mech. Eng., Nov. 11-16. A technique for connecting capillary tubing to a microfluidic chip that permits limited control over the placement of the seal is described by Bings et al., 1999, Analytical Chemistry, Vol. 71, pages 3292-6.
Coupling optical systems to fluidic systems has the challenges of both optical and fluidic engineering—precise alignment, watertight seals, low dead volumes, low dispersion, and efficient optical coupling are required. Optical coupling has been achieved both with remotely positioned light sources and detectors as well as with integrated optical lightguides. Remotely positioned light sources and detectors are most intolerant of relative movements of components, and thus require precise and stable positioning of all components. Integrated optical lightguides require expensive fabrication techniques and, unless the light source and detector are integrated into the MEMS device, optical coupling of the integrated lightguides in the MEMs device to external light sources and sensors is still required.
Summarily, it is desirable to provide simple, reliable, and manufacturable techniques for connecting fiber optics and capillaries to microfluidic MEMS devices.